Laser remote sensing can either be used to sense the particulars about remote solid objects (and as such is called laser radar), or it can be used to sense spectroscopic or aerosol returns (and as such is called lidar which stands for light detection and ranging). There are several methods of remote laser detection involving lidar. These methods include direct detection, absorption, florescence, differential absorption, and Raman scattering which are known to those skilled in the art. Some of the limiting factors to lidar which include wavelength adjustability or agility (most presently used lasers have a fixed frequency); and the general size and weight of the laser equipment. Of the above, the wavelength agility presents the greatest challenges.
The first attempt to overcome the wavelength agility problem included the use of dye lasers. These units, however, require a pump laser (and the dyes used as the lasing medium are toxic). These lasers are also more difficult to adjust than may be desired.
Diode lasers have also been Utilized in an attempt to solve the wavelength agility problem. These lasers, however, have limited tunability. Also, in the infrared range diode lasers must be cryogenically cooled which further complicates these lasers and makes them more expensive.
The third technique attempted to overcome this problem uses an optical parametric oscillator (OPO) as the frequency agile source. OPOs require a pump laser which use flashlamp pumped solid state lasing materials such as Nd:YAG. These pump sources are large and heavy and the beam quality required to pump the parametric oscillators satisfactorily are difficult to achieve, resulting in overall poor parametric oscillator performance.
The receiver in the above lidars typically use filters to separate the transmitted wavelengths. However, with a tunable source, a corresponding tunable receiver is required. In the past, spectrometers have provided the spectral dispersion required by tunable systems. Spectrometers, however, tend to be quite heavy and delicate.